1. Field of the Invention
The present invention relates to a data communication system, a data communication method and a data communication apparatus. More particularly, the invention relates to the network capable of communicating at high speeds, while carrying information data (including image data) intermixed with command data. The invention also relates to the communication protocol applicable to such network.
2. Related Background Art
Conventionally, among the peripheral devices of a personal computer (hereinafter referred to as a PC), a hard disc and a printer are those which have been used most frequently. These peripheral devices are connected with a PC through a digital interface, such as the input/output interface dedicated for use of a specific device, the SCSI (small computer system interface), or some other versatile interfaces.
Meanwhile, in recent years, the AV (audio/visual) equipment, such as a digital camera, a digital video camera, has been given more attention as one of the PC peripheral devices. These AV equipment are also connected with a PC through the interface dedicated for its specific use.
FIG. 1 is a view which shows the conventional communication system formed by the PC and AV equipment.
In FIG. 1, a reference numeral 101 designates an AV equipment (here, a digital camera); 102, a PC; and 103, a printer.
For the digital camera 101, a reference numeral 104 designates a memory that records the digital images after compression; 105, a decoding unit that expands and decodes the compressed image data thus recorded on the memory 104; 106, an image processing unit; 107, a D/A converter; 108, a display unit formed by EVF; and 109, the dedicated digital I/O unit that connects the digital camera 101 and the PC 102.
For the PC 102, a reference numeral 110 designates the dedicated digital I/O unit that connects the PC 102 and the digital camera 101; 111, an operation unit formed by a keyboard, a mouse, and the like; 112, a decoding unit that expands and decodes the compressed image data; 113, a display unit; 114, a hard disc; 115, a RAM and other memory; 116, an MPU; 117, a PCI bus; 118, a SCSI interface that connects the PC 102 and a printer 103.
For the printer 103, a reference numeral 119 designates the SCSI that connects the printer 103 and the PC 102; 120, a memory; 121, the printer head; 122, a printer controller that controls the operation of the printer 103; and 123, the printer driver.
For the conventional communication system, there is no compatibility between the digital interface (the digital I/O unit 109) having the digital cameral 101 and the digital interface (SCSI interface 110) having the printer 103, making it impossible to connect them directly. Therefore, if it is desired to communicate a still image from the digital camera 101 to the printer 103, for example, there is a need for the intervention of a PC under any circumstances.
Also, particularly when still images provided by the AV equipment or a large amount of data, such as moving images, should be handled by the dedicated interface or the SCSI interface, there are such problems as given below, among many others.
The data transfer rate is low.
The communication cable is thick due to the parallel communication.
The numbers and kinds of peripheral equipment that can be connected are limited.
The method of connection is restricted.
The data transfer cannot be made on real time.
In order to solve these problems, there is known IEEE (The Institute of Electrical and Electronics Engineers, Inc.) 1394-1995 standards as one of the higher speed and higher performance next-generation digital interfaces.
The digital interface in accordance with the IEEE 1394-1995 standards (hereinafter referred to as the 1394 interface) has the following distinctive features:
(1) The digital transfer speed is high.
(2) Both the real-time data transfer (that is, Isochronous transfer method) and the Asynchronous transfer method are supported.
(3) The connecting structure (topology) is possible with a higher degree of freedom.
(4) The plug and play function and the active-line insertion and deletion function are supported.
However, although the IEEE 1394-1995 standards define the physical and electrical structures, the most fundamental two data transfer methods, and the like, there are no definition as to the transmission and reception with respect to the kinds of data, the data formats, and the communication protocol based upon which the corresponding communication should be made.
Also, for the Isochronous transfer of the IEEE 1394-1995 standards, there is no regulation as to the response to the send-out packet. Therefore, it is not guaranteed whether or not each of the Isochronous packets is received exactly. As a result, if it is desired to transfer a plurality of continuous data exactly or if it is desired to transfer data exactly by dividing one file data into a plurality of data, the Isochronous transfer method cannot be adopted.
Also, the Isochronous transfer method of the IEEE 1394-1995 standards sets a limit of the total communication numbers to 64 even when there is an empty transfer band. As a result, if it is desired to perform a number of communications with a smaller transfer band, the Isochronous transfer method cannot be adopted.
Also, it is required to suspend the data transfer when the node power supply is turned on and off, the bus reset takes place corresponding to the connection/disconnection of the node, or the like. However, in accordance with the IEEE 1394-1995 standards, if the data transfer is suspended due to the bus reset or transfer errors, there is no way to know the contents of the data that are lost. Further, it is necessary to take an extremely complicated procedure in order to restore the transfer that has been once suspended.
Here, the bus reset means the function with which to recognize a new topology and automatically set the address (node ID) allocated to each of the nodes. With this function, it is made possible for the IEEE 1394-1995 standards to provide both the plug and play function and the active-line insertion and deletion function.
Also, for the system which is formed in accordance with the IEEE 1394-1995 standards, the real-time capability is not a requisite, but no specific proposal has been made as to the communication protocol which is needed for the continuous transfer of the comparatively large amount of object data which should be executed reliably, (such as data on still images, graphics data, text data, file data, program data) by dividing them into one or more segmental data for the intended transfer.
Also, for the communication system based on the IEEE 1394-1995 standards, there is no specific proposal as to the communication protocol which is needed for the implementation of data communication between a plurality of equipment using the communication method whereby to broadcast data asynchronously.